1. Field of the Invention
The present invention relates to a reception apparatus, a reception method, a program, and a reception system. More particularly, the invention relates to a reception apparatus, a reception method, a program, and a reception system for performing decoding processes faster than before.
2. Description of the Related Art
Recent years have witnessed widespread use of the modulation method called OFDM (Orthogonal Frequency Division Multiplexing) for transmitting digital signals. The OFDM method involves preparing numerous orthogonally oriented subcarriers within a transmission band in such a manner that data is allotted to the amplitude and phase of each of the subcarriers subject to digital modulation through PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation).
The OFDM method is often applied to terrestrial digital broadcasts that are highly vulnerable to multi-pass interference. The terrestrial digital broadcasts adopting the OFDM method typically comply with such standards as DVB-T (Digital Video Broadcasting-Terrestrial) and ISDB-T (Integrated Services Digital Broadcasting-Terrestrial).
Meanwhile, DVB-T2 (Digital Video Broadcasting-Terrestrial 2; second generation digital terrestrial television broadcasting system) is being worked out as a standard for the digital terrestrial broadcasts that utilize the OFDM method. DVB-T2 is described illustratively in the so-called Bluebook (DVB Bluebook A122, “Frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2),” DVB Document A122 June 2008, called the Non-patent Document 1 hereunder).
Under DVB-T2 (i.e., as stipulated in the Bluebook), frames each called a T2 frame are defined. Data is transmitted in units of a T2 frame. The T2 frame has two preamble signals called P1 and P2. These preamble signals include information necessary for processing the OFDM signal illustratively such as through decoding.
FIG. 1 is a schematic view showing the format of the T2 frame. The T2 format includes a P1 symbol, a P2 symbol, and data symbols, in that order. The P1 symbol is a symbol for transmitting P1 signaling that includes a transmission type and basic transmission parameters.
More specifically, the P1 signaling includes parameters S1 and S2. The parameters S1 and S2 indicate a P2 transmission type (i.e., whether P2 is transmitted in SISO (Single Input Single Output) fashion (using one transmitting antenna and one receiving antenna) or in MISO (Multiple Input Single Output) fashion (using multiple transmitting antennas but one receiving antenna)), and an FFT size for performing FFT computation of P2 (i.e., number of samples (symbols) subject to one FFT computation). It follows that to demodulate the P2 symbol requires demodulating the P1 symbol first.
The P2 symbol is a symbol for transmitting L1 pre-signaling and L1 post-signaling. The L1 pre-signaling includes information used by a reception apparatus receiving T2 frames for receiving and decoding the L1 post-signaling. The L1 post-signaling includes parameters needed by the reception apparatus for accessing the physical layer (i.e., its layer pipes).
DVB-T2 adopts a method called M-PLP (Multiple PLP (Physical Layer Pipe)). According to the M-PLP method, data is transmitted using packet sequences (data packet sequences) constituted by a plurality of Data PLP's composed of the packets left behind after extracting from a plurality of original transport streams (TS's) those packets (information) common to all TS's, and packet sequences (common packet sequences) called Common PLP's constituted by the packets common to the TS's. In other words, a Common PLP is composed of packets common to a plurality of TS's, and a Data PLP is constituted by packets unique to each of a plurality of TS's. On the receiving side, a single original TS is restored from a Common PLP and Data PLP's.
A Data PLP is a unit of service information, and a Common PLP is a portion common to at least two Data PLP's. Thus there exists the following relationship:Data PLP count≧2×Common PLP count≧0It follows that a plurality of Data PLP's are combined with a single Common PLP. At least two Data PLP's exist with regard to a given Common PLP, and there is one Common PLP for a given Data PLP.
To decode one unit of service information requires decoding two PLP's (a Common PLP and a Data PLP) simultaneously. The decoding information necessary for decoding the Common PLP and Data PLP (the information is called PLP information hereunder) is included in the L1 post-signaling. Thus when a decoding process is to be carried out, the PLP information made up of a Common PLP and Data PLP's corresponding to the service information desired to be decoded needs to be extracted from the L1 post-signaling.
As shown in FIG. 1, a plurality of units of PLP information are arranged in order of their PLPID's within the L1 post-signaling. Each unit of PLP information contains a PLPID for uniquely identifying each PLP, a PLP type for indicating whether this PLP is a Data PLP or a Common PLP, a GroupID for uniquely identifying the corresponding Common PLP and Data PLP, and PLP transmission parameter information.
Through these units of PLP information, a search is made for the PLP information composed of a Common PLP and Data PLP's corresponding to the target service information (this process is called the PLP search process). In FIG. 1, a search is made for Data PLP's for PLPID=2 and a Common PLP for PLPID=3 under the same GroupID=1. Then selection is made of the Data PLP's and Common PLP within the data symbols corresponding to the selected Data PLP's (PLPID=2) and appended (i.e., relevant) Common PLP (PLPID=3), so that the PLP's applicable to the target service information are decoded.
The PLP search process above is explained below in more detail by reference to FIGS. 2 through 4. As shown in FIG. 2, suppose that PLPID=4 is designated by a user's operation as the ID for the target service information (PLPID). In this case, a search is made starting from the first PLPID=0, until a Data PLP is found in a fifth PLPID=4 coinciding with the designated PLPID. Because GroupID=2 is allotted to the Data PLP's of PLPID=4, another search is made for a Common PLP which follows PLPID=4 and to which GroupID=2 is allotted. Then a Common PLP for a sixth PLPID=5 from the beginning is found, whereby the PLP information applicable to PLPID=5 is identified.
In another example in FIG. 3 where the Common PLP to which GroupID=2 is allotted is not PLPID=5 but PLPID=3, a search is made until the last PLP (PLPID=8) of the current T2 frame (i.e., its L1 post-signaling) is reached but no Common PLP of GroupID=2 is identified. In this case, the next T2 frame (its L1 post-signaling) is awaited, and a search is again made for a Common PLP to which GroupID=2 is allotted from among the Common PLP's ranging from the first PLP (PLPID=0) to the designated PLPID=4. In FIG. 3, GroupID=2 is found allotted to the Common PLP of PLPID=3, so that the PLP information applicable to PLPID=3 is identified.
In yet another example shown in FIG. 4, there is no Common PLP to which GroupID=2 applicable to the designated PLPID=4 corresponds. In this case, a search is made from PLPID=5 up to the last PLP of the current T2 frame (PLPID=8) and another search is made from the first PLP of the next T2 frame (PLPID=2) up to the designated PLPID=4, and yet no Common PLP of GroupID=2 is identified. This is a case where no Common PLP is defined.
As described, whereas the PLP search process is completed within one T2 frame in the case of FIG. 2, the process does not end within one T2 frame in the cases of FIGS. 3 and 4. Since the PLP search process needs to be continued into the next T2 frame, it takes longer to make the search in the cases of FIGS. 3 and 4 than in the case of FIG. 2.